Progressive Extended Compression Mask for Dynamic Trace

ABSTRACT

This invention provides trace address compression by comparing respective bytes of a current trace address with a stored prior trace address. Only the least significant bytes of the current trace address that do not match the stored prior trace address or are less significant than any section of the current trace address that does not match the stored prior trace address are transmitted. This sometimes reduces the amount of data that needs to be transmitted. The prior trace address may be updated with the current trace address if there is a complete mismatch.

TECHNICAL FIELD OF THE INVENTION

The technical field of this invention is emulation hardware particularlyfor highly integrated digital signal processing systems.

BACKGROUND OF THE INVENTION

Advanced wafer lithography and surface-mount packaging technology areintegrating increasingly complex functions at both the silicon andprinted circuit board level of electronic design. Diminished physicalaccess to circuits for test and emulation is an unfortunate consequenceof denser designs and shrinking interconnect pitch. Designed-intestability is needed so the finished product is both controllable andobservable during test and debug. Any manufacturing defect is preferablydetectable during final test before a product is shipped. This basicnecessity is difficult to achieve for complex designs without takingtestability into account in the logic design phase so automatic testequipment can test the product.

In addition to testing for functionality and for manufacturing defects,application software development requires a similar level of simulation,observability and controllability in the system or sub-system designphase. The emulation phase of design should ensure that a system of oneor more ICs (integrated circuits) functions correctly in the endequipment or application when linked with the system software. With theincreasing use of ICs in the automotive industry, telecommunications,defense systems, and life support systems, thorough testing andextensive real-time debug becomes a critical need.

Functional testing, where the designer generates test vectors to ensureconformance to specification, still remains a widely used testmethodology. For very large systems this method proves inadequate inproviding a high level of detectable fault coverage. Automaticallygenerated test patterns are desirable for full testability, andcontrollability and observability. These are key goals that span thefull hierarchy of test from the system level to the transistor level.

Another problem in large designs is the long time and substantialexpense involved in design for test. It would be desirable to havetestability circuitry, system and methods that are consistent with aconcept of design-for-reusability. In this way, subsequent devices andsystems can have a low marginal design cost for testability, simulationand emulation by reusing the testability, simulation and emulationcircuitry, systems and methods that are implemented in an initialdevice. Without a proactive testability, simulation and emulation plan,a large amount of subsequent design time would be expended on testpattern creation and upgrading.

Even if a significant investment were made to design a module to bereusable and to fully create and grade its test patterns, subsequent useof a module may bury it in application specific logic. This would makeits access difficult or impossible. Consequently, it is desirable toavoid this pitfall.

The advances of IC design are accompanied by decreased internalvisibility and control, reduced fault coverage and reduced ability totoggle states, more test development and verification problems,increased complexity of design simulation and continually increasingcost of CAD (computer aided design) tools. In the board design the sideeffects include decreased register visibility and control, complicateddebug and simulation in design verification, loss of conventionalemulation due to loss of physical access by packaging many circuits inone package, increased routing complexity on the board, increased costsof design tools, mixed-mode packaging, and design for produceability. Inapplication development, some side effects are decreased visibility ofstates, high speed emulation difficulties, scaled time simulation,increased debugging complexity, and increased costs of emulators.Production side effects involve decreased visibility and control,complications in test vectors and models, increased test complexity,mixed-mode packaging, continually increasing costs of automatic testequipment and tighter tolerances.

Emulation technology utilizing scan based emulation and multiprocessingdebug was introduced more than 10 years ago. In 1988, the change fromconventional in circuit emulation to scan based emulation was motivatedby design cycle time pressures and newly available space for on-chipemulation. Design cycle time pressure was created by three factors.Higher integration levels, such as increased use of on-chip memory,demand more design time. Increasing clock rates mean that emulationsupport logic causes increased electrical intrusiveness. Moresophisticated packaging causes emulator connectivity issues. Today thesesame factors, with new twists, are challenging the ability of a scanbased emulator to deliver the system debug facilities needed by today'scomplex, higher clock rate, highly integrated designs. The resultingsystems are smaller, faster, and cheaper. They have higher performanceand footprints that are increasingly dense. Each of these positivesystem trends adversely affects the observation of system activity, thekey enabler for rapid system development. The effect is called“vanishing visibility.”

FIG. 1 illustrates the trend in visibility and control over time andgreater system integration. Application developers prefer the optimumvisibility level illustrated in FIG. 1. This optimum visibility levelprovides visibility and control of all relevant system activity. Thesteady progression of integration levels and increases in clock ratessteadily decrease the actual visibility and control available over time.These forces create a visibility and control gap, the difference betweenthe optimum visibility and control level and the actual level available.Over time, this gap will widen. Application development tool vendors arestriving to minimize the gap growth rate. Development tools software andassociated hardware components must do more with less resources and indifferent ways. Tackling this ease of use challenge is amplified bythese forces.

With today's highly integrated System-On-a-Chip (SOC) technology, thevisibility and control gap has widened dramatically over time.Traditional debug options such as logic analyzers and partitionedprototype systems are unable to keep pace with the integration levelsand ever increasing clock rates of today's systems. As integrationlevels increase, system buses connecting numerous subsystem componentsmove on chip, denying traditional logic analyzers access to these buses.With limited or no significant bus visibility, tools like logicanalyzers cannot be used to view system activity or provide the triggermechanisms needed to control the system under development. A loss ofcontrol accompanies this loss in visibility, as it is difficult tocontrol things that are not accessible.

To combat this trend, system designers have worked to keep these busesexposed. Thus the system components were built in a way that enabled theconstruction of prototyping systems with exposed buses. This approach isalso under siege from the ever-increasing march of system clock rates.As the central processing unit (CPU) clock rates increase, chip to chipinterface speeds are not keeping pace. Developers find that apartitioned system's performance does not keep pace with its integratedcounterpart, due to interface wait states added to compensate forlagging chip to chip communication rates. At some point, thisperformance degradation reaches intolerable levels and the partitionedprototype system is no longer a viable debug option. In the current eraproduction devices must serve as the platform for applicationdevelopment.

Increasing CPU clock rates are also limiting availability of othersimple visibility mechanisms. Since the CPU clock rates can exceed themaximum I/O state rates, visibility ports exporting information innative form can no longer keep up with the CPU. On-chip subsystems arealso operated at clock rates that are slower than the CPU clock rate.This approach may be used to simplify system design and reduce powerconsumption. These developments mean simple visibility ports can nolonger be counted on to deliver a clear view of CPU activity. Asvisibility and control diminish, the development tools used to developthe application become less productive. The tools also appear harder touse due to the increasing tool complexity required to maintainvisibility and control. The visibility, control, and ease of use issuescreated by systems-on-a-chip tend to lengthen product developmentcycles.

Even as the integration trends present developers with a tough debugenvironment, they also present hope that new approaches to debugproblems will emerge. The increased densities and clock rates thatcreate development cycle time pressures also create opportunities tosolve them. On-chip, debug facilities are more affordable than everbefore. As high speed, high performance chips are increasingly dominatedby very large memory structures, the system cost associated with therandom logic accompanying the CPU and memory subsystems is dropping as apercentage of total system cost. The incremental cost of severalthousand gates is at an all time low. Circuits of this size may in somecases be tucked into a corner of today's chip designs. The incrementalcost per pin in today's high density packages has also dropped. Thismakes it easy to allocate more pins for debug. The combination ofaffordable gates and pins enables the deployment of new, on-chipemulation facilities needed to address the challenges created bysystems-on-a-chip.

When production devices also serve as the application debug platform,they must provide sufficient debug capabilities to support time tomarket objectives. Since the debugging requirements vary with differentapplications, it is highly desirable to be able to adjust the on-chipdebug facilities to balance time to market and cost needs. Since theseon-chip capabilities affect the chip's recurring cost, the scalabilityof any solution is of primary importance. “Pay only for what you need”should be the guiding principle for on-chip tools deployment. In thisnew paradigm, the system architect may also specify the on-chip debugfacilities along with the remainder of functionality, balancing chipcost constraints and the debug needs of the product development team.

FIG. 2 illustrates an emulator system 100 including four emulatorcomponents. These four components are: a debugger application program110; a host computer 120; an emulation controller 130; and on-chip debugfacilities 140. FIG. 2 illustrates the connections of these components.Host computer 120 is connected to an emulation controller 130 externalto host 120. Emulation controller 130 is also connected to target system140. The user preferably controls the target application on targetsystem 140 through debugger application program 110.

Host computer 120 is generally a personal computer. Host computer 120provides access the debug capabilities through emulator controller 130.Debugger application program 110 presents the debug capabilities in auser-friendly form via host computer 120. The debug resources areallocated by debug application program 110 on an as needed basis,relieving the user of this burden. Source level debug utilizes the debugresources, hiding their complexity from the user. Debugger applicationprogram 110 together with the on-chip trace and triggering facilitiesprovide a means to select, record, and display chip activity ofinterest. Trace displays are automatically correlated to the source codethat generated the trace log. The emulator provides both the debugcontrol and trace recording function.

The debug facilities are preferably programmed using standard emulatordebug accesses through a JTAG or similar serial debug interface. Sincepins are at a premium, the preferred embodiment of the inventionprovides for the sharing of the debug pin pool by trace, trigger, andother debug functions with a small increment in silicon cost. Fixed pinformats may also be supported. When the pin sharing option is deployed,the debug pin utilization is determined at the beginning of each debugsession before target system 140 is directed to run the applicationprogram. This maximizes the trace export bandwidth. Trace bandwidth ismaximized by allocating the maximum number of pins to trace.

The debug capability and building blocks within a system may vary.Debugger application program 100 therefore establishes the configurationat runtime. This approach requires the hardware blocks to meet a set ofconstraints dealing with configuration and register organization. Othercomponents provide a hardware search capability designed to locate theblocks and other peripherals in the system memory map. Debuggerapplication program 110 uses a search facility to locate the resources.The address where the modules are located and a type ID uniquelyidentifies each block found. Once the IDs are found, a design databasemay be used to ascertain the exact configuration and all system inputsand outputs.

Host computer 120 generally includes at least 64 Mbytes of memory and iscapable of running Windows 95, SR-2, Windows NT, or later versions ofWindows. Host computer 120 must support one of the communicationsinterfaces required by the emulator. These may include: Ethernet 10T and100T, TCP/IP protocol; Universal Serial Bus (USB); Firewire IEEE 1394;and parallel port such as SPP, EPP and ECP.

Host computer 120 plays a major role in determining the real-time dataexchange bandwidth. First, the host to emulator communication plays amajor role in defining the maximum sustained real-time data exchangebandwidth because emulator controller 130 must empty its receivereal-time data exchange buffers as fast as they are filled. Secondly,host computer 120 originating or receiving the real-time data exchangedata must have sufficient processing capacity or disc bandwidth tosustain the preparation and transmission or processing and storing ofthe received real-time data exchange data. A state of the art personalcomputer with a Firewire communication channel (IEEE 1394) is preferredto obtain the highest real-time data exchange bandwidth. This bandwidthcan be as much as ten times greater performance than other communicationoptions.

Emulation controller 130 provides a bridge between host computer 120 andtarget system 140. Emulation controller 130 handles all debuginformation passed between debugger application program 110 running onhost computer 120 and a target application executing on target system140. A presently preferred minimum emulator configuration supports allof the following capabilities: real-time emulation; real-time dataexchange; trace; and advanced analysis.

Emulation controller 130 preferably accesses real-time emulationcapabilities such as execution control, memory, and register access viaa 3, 4, or 5 bit scan based interface. Real-time data exchangecapabilities can be accessed by scan or by using three higher bandwidthreal-time data exchange formats that use direct target to emulatorconnections other than scan. The input and output triggers allow othersystem components to signal the chip with debug events and vice-versa.Bit I/O allows the emulator to stimulate or monitor system inputs andoutputs. Bit I/O can be used to support factory test and other lowbandwidth, non-time-critical emulator/target operations. Extendedoperating modes are used to specify device test and emulation operatingmodes. Emulator controller 130 is partitioned into communication andemulation sections. The communication section supports hostcommunication links while the emulation section interfaces to thetarget, managing target debug functions and the device debug port.Emulation controller 130 communicates with host computer 120 using oneof industry standard communication links outlined earlier herein. Thehost to emulator connection is established with off the shelf cablingtechnology. Host to emulator separation is governed by the standardsapplied to the interface used.

Emulation controller 130 communicates with the target system 140 througha target cable or cables. Debug, trace, triggers, and real-time dataexchange capabilities share the target cable, and in some cases, thesame device pins. More than one target cable may be required when thetarget system 140 deploys a trace width that cannot be accommodated in asingle cable. All trace, real-time data exchange, and debugcommunication occurs over this link. Emulator controller 130 preferablyallows for a target to emulator separation of at least two feet. Thisemulation technology is capable of test clock rates up to 50 MHZ andtrace clock rates from 200 to 300 MHZ, or higher. Even though theemulator design uses techniques that should relax target system 140constraints, signaling between emulator controller 130 and target system140 at these rates requires design diligence. This emulation technologymay impose restrictions on the placement of chip debug pins, boardlayout, and requires precise pin timings. On-chip pin macros areprovided to assist in meeting timing constraints.

The on-chip debug facilities offer the developer a rich set ofdevelopment capability in a two tiered, scalable approach. The firsttier delivers functionality utilizing the real-time emulation capabilitybuilt into a CPU's mega-modules. This real-time emulation capability hasfixed functionality and is permanently part of the CPU while the highperformance real-time data exchange, advanced analysis, and tracefunctions are added outside of the core in most cases. The capabilitiesare individually selected for addition to a chip. The addition ofemulation peripherals to the system design creates the second tierfunctionality. A cost-effective library of emulation peripheralscontains the building blocks to create systems and permits theconstruction of advanced analysis, high performance real-time dataexchange, and trace capabilities. In the preferred embodiment fivestandard debug configurations are offered, although customconfigurations are also supported. The specific configurations arecovered later herein.

SUMMARY OF THE INVENTION

Trace streams collect native program counter or data reference addressesas part of a synchronization marker or when encoding register branchesor exception discontinuities. Particular system configurations may mapcode to different physical locations; but locality of the code or datawithin certain system/application configurations then tend to be fairlystatic.

Dynamic trace stream=s bandwidth is limited by pin allocation andsignificantly by the presence of register branches, exceptions andsynchronization markers (which all use native program counter tags).Dynamic trace streams which collect data addresses as part of datalogging or address locality profiling require frequent exporting ofthese addresses, without adversely intruding into system performance.This invention enables data compression of exported program counter ordata reference addresses by a byte wise comparison with a prior address.Only differing bytes are included in the trace stream.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of this invention are illustrated in thedrawings, in which:

FIG. 1 illustrates the visibility and control of typical integratedcircuits as a function of time due to increasing system integration;

FIG. 2 illustrates an emulation system to which this invention isapplicable;

FIG. 3 illustrates in block diagram form a typical integrated circuitemploying configurable emulation capability;

FIG. 4 illustrates a flow chart of the compare operation of theprogressive extended compression mask of this invention; and

FIG. 5 illustrates in block diagram form a circuit including both theprogressive and the programmable compression masks.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The problem solved by this invention is dealing with tracing addressreferences. Microprocessors and digital signal processors typically havearchitectures which run at high clock rates and generate large amountsof trace information. Processing such trace information by traceacquisition and encoding hardware without adversely affecting theaccuracy of profiling is difficult. Due to the high demand for bandwidthon external dedicated trace export pins at the chip boundary, tracehardware may be forced to stall the central processing unit in order toguarantee collection of all trace information. This makes profiling ofcentral processing unit performance using the same trace hardwareintrusive and hence very inaccurate.

A flexible compression scheme is needed to deal with the collection ofthese reference addresses. This invention provides a progressivecompression scheme for base addresses used by the application togetherwith a way of exporting only an offset from this address. This schemeutilizes temporal and spatial locality of reference addresses to reducethe amount of data exported via the pins in situations where ranges ofaddresses fall within a common base address.

This invention is a progressive extended compression mask for dynamictrace. This invention reduces the bandwidth requirement when dealingwith encoding register branches, data address references, exceptions orexporting synchronization markers. An example implementation of such amask, to deal with a program counter or data address of 32 bits,includes a 24-bit register which is holds the last valid program counteraddress. The 24-bit mask is used to compare the upper range of thenative program counter or data address as it emerges from the output ofthe pipeline flattener within the trace acquisition hardware. The maskis compared byte by byte and decisions are made as to how much of thereference address needs to be exported on to the trace stream.

FIG. 3 illustrates an example of one on-chip debug architectureembodying target system 140. The architecture uses several moduleclasses to create the debug function. One of these classes is eventdetectors including bus event detectors 210, auxiliary event detectors211 and counters/state machines 213. A second class of modules istrigger generators including trigger builders 220. A third class ofmodules is data acquisition including trace collection 230 andformatting. A fourth class of modules is data export including traceexport 240, and real-time data exchange export 241. Trace export 240 iscontrolled by clock signals from local oscillator 245. Local oscillator245 will be described in detail below. A final class of modules is scanadaptor 250, which interfaces scan input/output to CPU core 201. Finaldata formatting and pin selection occurs in pin manager and pin micros260.

The size of the debug function and its associated capabilities for anyparticular embodiment of a system-on-chip may be adjusted by eitherdeleting complete functions or limiting the number of event detectorsand trigger builders deployed. Additionally, the trace function can beincrementally increased from program counter trace only to programcounter and data trace along with ASIC and CPU generated data. Thereal-time data exchange function may also be optionally deployed. Theability to customize on-chip tools changes the application developmentparadigm. Historically, all chip designs with a given CPU core werelimited to a fixed set of debug capability. Now, an optimized debugcapability is available for each chip design. This paradigm change givessystem architects the tools needed to manage product development risk atan affordable cost. Note that the same CPU core may be used withdiffering peripherals with differing pin outs to embody differingsystem-on-chip products. These differing embodiments may requirediffering debug and emulation resources. The modularity of thisinvention permits each such embodiment to include only the necessarydebug and emulation resources for the particular system-on-chipapplication.

The real-time emulation debug infrastructure component is used to tacklebasic debug and instrumentation operations related to applicationdevelopment. It contains all execution control and register visibilitycapabilities and a minimal set of real-time data exchange and analysissuch as breakpoint and watchpoint capabilities. These debug operationsuse on-chip hardware facilities to control the execution of theapplication and gain access to registers and memory. Some of the debugoperations which may be supported by real-time emulation are: setting asoftware breakpoint and observing the machine state at that point;single step code advance to observe exact instruction by instructiondecision making; detecting a spurious write to a known memory location;and viewing and changing memory and peripheral registers.

Real-time emulation facilities are incorporated into a CPU mega-moduleand are woven into the fabric of CPU core 201. This assures designsusing CPU core 201 have sufficient debug facilities to support debuggerapplication program 110 baseline debug, instrumentation, and datatransfer capabilities. Each CPU core 201 incorporates a baseline set ofemulation capabilities. These capabilities include but are not limitedto: execution control such as run, single instruction step, halt andfree run; displaying and modifying registers and memory; breakpointsincluding software and minimal hardware program breakpoints; andwatchpoints including minimal hardware data breakpoints.

This invention concerns data compression for data and program counteraddresses. In the preferred embodiment this invention is incorporatedinto trace collection 230. This invention compares the current addresswith a prior address and transmits only differing address bytes.

FIG. 4 illustrates this process in flow chart form. Process 400 beingsat start block 401. Process 400 first compares byte 3 of the programcounter or data address with the 24-bit mask (processing block 402). Ifthese bytes are not equal (No at decision block 403), then process 400exports all four bytes (bytes 3 to 0, 32 bits) of the address(processing block 404). If these bytes are equal (Yes at decision block403), then process 400 compares the byte 2 of the address with the mask(processing block 405). If these bytes are not equal (No at decisionblock 406), then process 400 exports the three least significant bytes(bytes 2 to 0, 24 bits) of the address (processing block 407). If thesebytes are equal (Yes at decision block 406), then process 400 comparesbyte 1 of the address and the mask (processing block 408). If thesebytes are not equal (No at decision block 409), then process 400 exportsall the two least significant bytes (bytes 1 and 0, 16 bits) of theaddress (processing block 410). If these bytes are equal (Yes atdecision block 409), then process 400 exports only the least significantbyte (byte 0, 8 bits) of the address (processing block 411).

The progressive nature of this mask takes advantage if addresses aregenerated within a particular range of locations to reduce the number ofbytes that need to be sent with each reference. An initial referenceaddress or program counter value will have been sent containing all 4bytes. If subsequent register branches or data references meet any textconditions of FIG. 4, this scheme will always send out less than themaximum number of bytes. Locality of data addresses and program codemake this scheme more efficient in terms of preservation of bandwidthwhen dealing with profiling data addresses or program counter addresseswith timing. New reference addresses are compared byte by byte with theprior valid reference address contained in the storage register.

A similar technique can be used for a programmable extended compressionmask. Rather than compare the current data or program counter addresswith a prior address, the comparison is made with a predeterminedaddress supplied by the central processing unit. This comparison addressis preferably specified by a memory mapped data register. Theprogrammable nature of this mask permits information known concerningthe location of code and data to be used to help with the compression ofthese addresses. This scheme permits intelligent determination ahead ofthe trace collection session of which data and code locations are likelyto be referenced in the execution of the application or segment of codewithin it. Plural memory mapped comparison registers can be used to dealwith more section descriptions for data and code. This can deal with awider range of data addresses and code. In the case of plural compareregisters, an index associated with the compare register hit by thecompare is provided along with the offset address when exporting thetrace data packets.

FIG. 5 illustrates a combined progressive and programmable compressionschemes for tracing addresses as part of a dynamic trace stream. Thetrace address is received via address input 501. Trace stream controlcircuit 503 supplies 8-bit encoded trace packets via output 502. Tracestream control circuit 503 receives the trace address, and a Byte1_validsignal from NOR gate 504, a Byte2 valid signal from NOR gate 505 and aByte#_valid signal from NOR gate 505. NOR gates 504 to 506 are driven bycomparison sections 510 and 520.

FIG. 5 illustrates progressive trace compare section 510. Byte 1 of thetrace address at input 501 supplies one input of Byte 1 comparator 511.Byte 2 of the trace address at input 501 supplies one input of byte2comparator 512. Byte 3 of the trace address at input 501 supplies oneinput to comparator 513. Comparators 511, 512 and 513 receivecorresponding bytes from comparison register 516. Each comparator 511,512 and 513 compares the 8 bits of their corresponding byte of the traceaddress received from input 501 and the compare address stored incomparison register 516. The respective comparison outputs of comparator511, 512 and 513 are supplied to one input of NOR gates 504, 504 and506.

Initially comparison address register 516 is empty, i.e. all 0's. Uponinitialization, the first trace address is stored in comparison addressregister 516 via multiplexer 515. The second trace address is comparedwith the first comparison address now stored in comparison addressregister 516. A match at comparator 513 will cause trace stream control503 not to output byte 3. A match at comparators 513 and 512 will causetrace stream control 503 not to output bytes 3 and 2. A match atcomparators 513, 512 and 511 will cause trace stream control 503 tooutput byte 0 only. Table 1 shows the output function of trace streamcontrol circuit 503. TABLE 1 Output Byte 1 Byte 2 Byte 3 Bytes — — Not3210 — — Match -210 — Match Match --10 Match Match Match ---0Note that “-” is a don't care entry. For example, if comparator 512 or511 detects a match but comparator 513 does not, then all four bytes areoutput.

The address stored in comparison address register 516 recirculates viathe “0” input of multiplexer 515 until changed. This would typicallytake place when all three comparator 511, 512 and 513 fail to detect amatch. If all the byte comparisons miss, then the trace address hasprobably moved to a different address region. It is best to reset thecomparison address to this new address region and make futurecomparisons with the reset compare address. A “1” valid_address signalcauses multiplexer 515 to select the trace address received on traceaddress input 501. Comparison register 516 stores this address selectedby multiplexer 515. The valid-address signal would normally be “1” for asingle cycle. Thereafter comparisons are made with the newly storedtrace address.

FIG. 5 illustrates programmable trace compare section 520, which issimilar to progressive trace compare section 510. Byte 1 of the traceaddress at input 501 supplies one input of Byte1 comparator 521. Byte 2of the trace address at input 501 supplies one input of byte2 comparator522. Byte 3 of the trace address at input 501 supplies one input tocomparator 523. Comparators 521, 522 and 523 receive corresponding bytesfrom comparison register 526. Each comparator 521, 522 and 523 comparesthe 8 bits of their corresponding byte of the trace address receivedfrom input 501 and the compare address stored in register 526. Therespective comparison outputs of comparator 521, 522 and 523 aresupplied to one input of NOR gates 504, 504 and 506.

The compare address stored in compare address register 526 ordinarilyrecirculates via the “0” input of multiplexer 525. The centralprocessing unit can change this comparison address by writing to thememory-mapped register. Upon a write to the memory-mapped registercorresponding to comparison address register 526, the address appears onwrite bus 527 and the Reg_write signal is a “1”. Multiplexer 525 selectsthe address on write bus 527 for storage in comparison address register526. The Reg_write signal would ordinarily be “1” for only a singlecycle or the minimum time for the write to complete. Thereafter,comparisons are made with the central processing unit supplied addressnow stored in comparison address register 526.

If there is more than one comparison unit, such a comparison units 510and 520 illustrated in FIG. 5, trace stream control circuit 503 requiressome manner (not illustrated) to identify the operative comparison unit.Trace stream compare circuit 503 places a comparison unit identitymarker in the output trace stream 502. The receive unit then has a meansof following the compressed trace stream. A new comparison address wouldbe stored in comparison address register 516 only following transmissionof the entire address to the receive unit. Control of the centralprocessing unit program enables the receive unit a priori knowledge ofthe central processing unit specified address stored in comparisonaddress register 526. Knowledge of which comparison unit is used wouldenable reconstruction of the trace address.

There are further complications if there are more than one progressivecomparison unit 510 or more than one programmable comparison unit 520.Plural programmable comparison units 520 cause the least problems. Theuser presumably has control of or at least knowledge of the centralprocessing unit specified trace addresses. Matching that knowledge tothe comparison unit identity marker is feasible.

Plural progressive comparison units 510 are more problematic. Sometechnique is needed in order to keep track of the data stored withineach comparison address register 516. This could done by trace streamcontrol circuit 503 explicitly indicating in the trace stream when acomparison address register 516 is loaded and the identity of thatregister. Alternatively, it could be assumed that the least recentlyused comparison address register is replaced whenever an full 32-bittrace address is transmitted. Keeping track of the contents of pluralcomparison address registers may enable greater trace stream density bypermitting two or more address regions to be compared.

This application describes a technique making comparisons bycorresponding address bytes. Those skilled in the art would recognizethat this represents merely a convenient design choice. The comparisonscould be made by nibbles (4 bits), words (16 bits) or any otherconvenient data length including individual bits. The comparison lengthselected is preferably an integral fraction of the length of theaddresses compared and in equal lengths throughout the addresses.Employing shorter comparison lengths increases the possibility ofenabling data compression at the expense of additional controlprocesses. Longer comparison lengths require less control processes butreduce the potential available data compression.

1 to
 8. (canceled)
 9. A trace compression apparatus comprising: an inputreceiving a current trace address; a trace address register storing aprior trace address; a plurality of section comparators, each sectioncomparator connected to said trace address register receiving a sectionof said prior trace address and connected to said input receiving acorresponding section of said current trace address, each sectioncomparator indicating a match or a not match of said correspondingsections; and a trace stream control connected to said input and each ofsaid section comparators, said trace stream control transmitting onlyleast significant sections of the current trace address that do notmatch respective sections of the stored prior trace address or are lesssignificant than a section of the current trace address that does notmatch the stored prior trace address.
 10. The trace compressionapparatus of claim 9, wherein: said trace stream control alwaystransmits a least significant section of said current trace address. 11.The trace compression apparatus of claim 9, further comprising: amultiplexer having a first input receiving an output of said traceaddress register, a second input receiving said current trace address,an output connected to an input of said trace address register, and acontrol input, said multiplexer coupling one of said first input or saidsecond input to said output dependent upon a signal at said controlinput; and said trace stream control connected to said control input ofsaid multiplexer, said trace stream control supplying a signal to saidcontrol input of said multiplexer causing said multiplexer to selectsaid second input thereby storing said current trace address in saidtrace address register if no section of said current trace addressmatches the corresponding section of said prior trace address.
 12. Thetrace address compression apparatus of claim 9, wherein: said section isan integral fraction of the length of the stored prior trace address andthe current trace address.
 13. The trace address compression apparatusof claim 9, wherein: said section has a length equal to 8 bits.
 14. Thetrace address compression apparatus of claim 9, wherein: said sectionhas a length equal to 4 bits.
 15. The trace address compressionapparatus of claim 9, wherein: said section has a length equal to 16bits.
 16. The trace address compression apparatus of claim 9, wherein:said prior trace address and said current address have length of 32bits; said plurality of section comparators consists of three bytecomparators comparing respective three most significant bytes of saidprior trace address and said current trace address; and said tracestream control operates to transmit four bytes of said current traceaddress if a most significant byte comparator does not detect a match,transmit three least significant bytes of said current trace address ifa most significant byte comparator detects a match and a second mostsignificant byte comparator does not detect a match, transmit two leasesignificant bytes of said current trace address if a most significantbyte comparator and a second most significant byte comparator eachdetect a match and a third most significant byte comparator does notdetect a match, and transmit a least significant byte of said currenttrace address if a most significant byte comparator, a second mostsignificant byte comparator and a third most significant byte comparatoreach detect a match.